Original calculator data
How many solar panels do you need by monthly kWh?
At five peak sun hours per day, Watt My Roof's model estimates about 11 panels for 500 kWh/month, 21 panels for 1,000 kWh/month and 31 panels for 1,500 kWh/month, using 400-watt panels and a 20% system-loss allowance.
Published August 23, 2026
Solar panels needed by monthly kWh
The table below uses 400-watt panels, a 0.80 performance factor and a 100% electricity offset. It compares monthly electricity use against four common sunlight assumptions: 3, 4, 5 and 6 peak sun hours per day.
| Monthly electricity use | 3 sun hrs/day | 4 sun hrs/day | 5 sun hrs/day | 6 sun hrs/day | System size at 5 sun hrs |
|---|---|---|---|---|---|
| 250 kWh/mo | 9 panels | 7 panels | 6 panels | 5 panels | 2.4 kW |
| 500 kWh/mo | 18 panels | 13 panels | 11 panels | 9 panels | 4.4 kW |
| 750 kWh/mo | 26 panels | 20 panels | 16 panels | 13 panels | 6.4 kW |
| 1,000 kWh/mo | 35 panels | 26 panels | 21 panels | 18 panels | 8.4 kW |
| 1,250 kWh/mo | 43 panels | 33 panels | 26 panels | 22 panels | 10.4 kW |
| 1,500 kWh/mo | 52 panels | 39 panels | 31 panels | 26 panels | 12.4 kW |
| 2,000 kWh/mo | 69 panels | 52 panels | 42 panels | 35 panels | 16.8 kW |
| 2,500 kWh/mo | 86 panels | 65 panels | 52 panels | 43 panels | 20.8 kW |
| 3,000 kWh/mo | 103 panels | 78 panels | 62 panels | 52 panels | 24.8 kW |
Download the modeled data as CSV: solar-panels-by-monthly-kwh.csv.
Quick reference at five peak sun hours
Five peak sun hours is a common middle planning assumption, but it is not a universal default. Use this section for fast comparison, then adjust the sunlight assumption for your location.
| Usage pattern | Monthly kWh | Estimated panels | Estimated system size |
|---|---|---|---|
| Efficient apartment or essentials-only backup | 250 kWh/mo | 6 panels | 2.4 kW |
| Smaller home or low-use household | 500 kWh/mo | 11 panels | 4.4 kW |
| Moderate home electricity use | 750 kWh/mo | 16 panels | 6.4 kW |
| Higher-use home with AC or electric loads | 1,000 kWh/mo | 21 panels | 8.4 kW |
| Large or all-electric home | 1,500 kWh/mo | 31 panels | 12.4 kW |
The formula behind this dataset
This page is intentionally formula-first. The goal is to make the calculator's assumptions visible enough for homeowners, search engines and AI answer engines to understand the output without a hidden lead form.
Annual output per panel used in the model
The same 400-watt panel produces different annual electricity depending on sunlight. That is why monthly kWh alone is not enough.
| Peak sun hours | Annual output per 400 W panel | What it means |
|---|---|---|
| 3 hours/day | 350 kWh/year | Lower production, higher panel count |
| 4 hours/day | 467 kWh/year | Lower production, higher panel count |
| 5 hours/day | 584 kWh/year | Middle planning assumption |
| 6 hours/day | 701 kWh/year | Higher production, lower panel count |
Watt My Roof publishes the assumptions instead of forcing every solar question through a quote form. This is still a planning model, not a permit-ready design, installer quote, savings guarantee or utility approval.
When monthly kWh can mislead you
- One high bill is not an annual average: summer air conditioning or winter electric heat can distort a single month.
- Future loads matter: an EV, heat pump, pool pump or electric water heater can raise your future kWh.
- Roof fit still matters: the table can say you need 31 panels while your usable roof only fits 23.
- Utility rules can cap the system: some utilities limit system size, exports or net-metering value.
FAQ
How many solar panels do I need based on monthly kWh?
Using 400-watt panels, 20% system losses and 100% electricity offset, a home using 500 kWh per month needs about 11 panels at five peak sun hours. A 1,000 kWh home needs about 21 panels, and a 1,500 kWh home needs about 31 panels.
Is monthly kWh better than estimating by home size?
Yes. Monthly kWh from your electric bill is usually better than square footage or bedroom count because it measures the actual electricity the solar system would need to replace.
Why does the same monthly kWh need different panel counts?
The same electricity target needs different panel counts because local sunlight changes annual output per panel. Lower peak sun hours require more panels; higher peak sun hours require fewer panels under the same assumptions.
Does this table prove what system I should buy?
No. This is modeled planning data, not an installation quote. Roof area, shade, panel wattage, utility rules, system losses, batteries and local permitting can all change the final design.
This is original modeled data from Watt My Roof's published assumptions: monthly electricity use, panel wattage, peak sun hours, system losses and desired offset. For the full context, see the methodology page, the peak sun hours data page and the roof-fit calculator.